PROJECT SUMMARY Little is known about the neural mechanisms that regulate natural dynamic cues during human social and emotional interactions, although these mechanisms are impaired in many psychiatric and neurological disorders. Although it is widely understood that social signals such as facial expressions carry salient, but implicit, emotional and social cues, these ?real-time? pathways have not been investigated with dual-brain neuroimaging techniques. This unmet need is largely due to technological limitations that prevent neuroimaging of two or more individuals during natural interactive situations. We overcome this technical ?roadblock? with recent advances in an emerging human brain imaging technology, functional near-infrared spectroscopy (fNIRS). This non-invasive technique detects active neural tissue based on hemodynamic signals measured by variations in the absorption spectra associated with oxyhemoglobin and de-oxyhemoglobin. Because detectors and emitters are surface mounted on the head, absent a high magnetic field, they are relatively insensitive to head movement and thus successfully applied to dyadic experiments. The focus of this proposal is to gain a comprehensive understanding of the mechanisms that underlie dynamic cross-brain neural coupling during real interpersonal interactions. Cross-brain neural coupling is defined as the correlation between the temporal patterns of the signals of two brains. It has been proposed that these matched patterns represent shared neural processes including dynamic exchanges of information. However, the basic assumptions of shared information and temporal resonance patterns between specific brain-to-brain regions has not been tested. We pioneer tests of these hypothesis using eye-to-eye contact as a metric of shared information and predict that dynamic neural coupling between the two brains will increase with increasing numbers of eye-to-eye contact events. Mimicry of facial expressions is also a metric of emotional contagion as well as shared information between brains. We further test the hypothesis that neural coupling will increase with the level of mimicry also by virtue of the shared information Confirmation of the hypothesis that neural coupling represents shared information between the two brains would provide a singular advance for understanding mechanisms for dynamic interactions. Both approaches include variations of emotive expressions to test the additional hypothesis that content as well as shared information might influence dynamic coupling mechanisms. Findings from these studies are expected to open a new direction for the study of live and dynamic interactions between individuals, and provide foundational components to a general framework for models of face-to-face interactions. A long-term goal is to understand the neural underpinnings of affective disorders as they present in clinically-relevant and real-world situations.